The present invention relates to vehicle roll control systems, and in particular to vehicle stability systems for detection and prevention of overturning of commercial vehicles, including commercial vehicle trailers.
Commercial vehicles, including the trailers of tractor-trailer vehicles, are frequently loaded with high centers of gravity. During tight turns at highway speeds, these vehicles can roll over. Reducing the vehicle speed when high lateral accelerations are detected or when a wheel is lifted can reduce the tendency for such a vehicle to rollover and improve highway safety.
As described in U.S. Patent Publication No. US 20040119335 A1, the risk of a vehicle overturning event may be estimated by monitoring transverse acceleration (i.e., acceleration lateral to the vehicle's longitudinal axis), and by monitoring vehicle wheel speed during the light application of braking force to the vehicle wheels, and assessing whether the wheels are off the ground or only lightly loaded in the normal direction depending on the resulting wheel speed change.
Roll estimation systems, such as that described in U.S. Patent Publication No. US 20040119335 A1, are based on braking system architectures in which two independent pressure channels exist, with two separate brake application modulators. These systems are derived from two-channel modulator anti-lock braking system (“ABS”) architectures, which are common in Europe. The two independent channels permit the independent application of the brakes on each side of the vehicle during the wheel speed monitoring portion of the roll-over risk assessment, so that differential wheel speed change comparisons may be made.
In contrast to European practice, North American trailer ABS systems are predominantly based on a single pressure channel system, in which a single brake application modulator serves the vehicle wheels on both sides of at least one vehicle axle, i.e., a “single channel” system. This is in part due to the additional costs associated with providing two independent channels. In the absence of a second independent pressure channel, it has been commonly assumed that the known two-channel roll assessment and prevention systems could not be used with single-channel North American-type commercial vehicles.
In addressing these and other objectives, the present invention provides a vehicle roll stability system architecture in which the roll stability function is performed with a single modulator controlling brake application on both left and right side wheels on one or more axles, thereby providing the benefits of the more complicated and more costly two-channel roll estimation systems in a simpler and lower cost system which is compatible with North American-type single channel commercial vehicle ABS systems.
This objective is met by providing a trailer roll stability system comprising a single-channel pressure modulator and wheel sensors (including single-channel architectures commonly known as “2s1m” (two sensors, one modulator) or “4s1m” (four sensors, one modulator)), and an electronic control system. The electronic control system evaluates vehicle speed, lateral acceleration and wheel speed change information, and controls brake pressure application during roll evaluation operation. However, unlike prior practice two-channel systems in which brake pressure is applied to different wheels independently, the electronic control system of the present invention controls the single brake pressure modulator to lightly apply the brakes simultaneously at all the wheels.
The Applicant has discovered through experimentation that contrary to previous assumptions, a single-channel braking system can be used with a roll detection and prevention system while providing anti-roll performance which is as good or better than comparable two-channel systems.
With the high lateral acceleration, the vehicle weight is transferred to the outside wheels and the inside wheels may lift off of the ground if the center of gravity is high. In a prior two-channel system, if there has been no application of a brake intervention (e.g., brake application by the vehicle operator or as a result of a brake system-commanded braking event), when there is a high lateral acceleration the two-channel system executes a “pressure test” mode, applying a small test pressure only to the brakes on the side of the vehicle inside the turn (i.e., using only the pressure modulator in the channel which controls the inside wheel brakes). If the small pressure applied in the one channel causes the inside wheels to lock, it may be assumed that the inside wheels are off of the ground or only very lightly loaded.
In the inventive single-channel system, it has been found that the small test pressure may be simultaneously applied to both the inside and outside wheels without significant detriment. Testing confirmed that in a vehicle such as a trailer equipped with a single brake pressure modulator controlling all the vehicle brakes, the test pressure application could be used to reliably determine whether the inside wheels were off of the ground or very lightly loaded, as the test pressure is too small to significantly affect the rotation of the outside wheels which were heavily loaded.
On the other hand, in the above-described two-channel system during a braking intervention the system applies the brakes in the channel controlling the brakes on the outside wheels, as the outside wheels carrying the greatest portion of the vehicle weight in a turn (these wheels therefore possessing the greatest capacity to slow the vehicle since the higher normal force on the outside wheels allows higher braking forces to be applied before the wheels lock-up and slide). Brake pressure is not applied to the channel for the inside wheels due to concerns with the potential for inside wheel lock-up and excessive tire wear (e.g., “flat-spotting”).
Where it has been previously assumed that pressure should only be applied to the outside wheels during braking intervention in two-channel braking systems, it has been discovered that the application of braking system pressure in a single-channel system to both the inside and outside wheels provides the desired anti-roll performance and permits roll detection testing may still be performed, without significant negative consequences.
During a braking event, rather than applying a test pressure to the inside wheels, the electronic control unit momentarily slightly reduces the applied brake pressure while monitoring the wheel speed of the inside wheels. If the inside wheel speeds remain at zero, it can be assumed that the wheels are off the ground. Alternatively, if the inside wheel speeds substantially increase, it can be assumed that the inside wheels are only lightly loaded as they were locked or nearly locked before the brake pressure was reduced. As a further alternative, if after the test pressure is removed (i.e., the brake pressure is restored to its pre-test value), the inside wheel speeds substantially decrease or decrease faster than the outer wheel speeds, it also can be assumed that the inside wheels are only lightly loaded as they were locked or nearly locked before the brake pressure was reduced.
On the other hand, if braking is not occurring, a slight test pressure may be applied by the single channel system to all of the wheels while monitoring the wheel speed of the inside wheels. If the wheel speed substantially decreases or decreases faster than the outer wheel speeds, it can be assumed that the wheels are off the ground or only lightly loaded. Alternatively, if after the test pressure is removed (i.e., the brake pressure is restored to its pre-test value of zero pressure), if the inside wheel speeds substantially increase or increase faster than the outer wheel speeds, it also can be assumed that the inside wheels are only lightly loaded but not off the ground as they were locked or nearly locked when the test brake pressure was applied.
With these observations, the existence of the potential for overturning may be deduced. It was noted that while the application of brake pressure in a single-channel system may result in momentary skidding of the inside wheels (due to the lack of independent operation possible with two-channel systems), the potential for increased tire wear or “flat-spotting” during braking is substantially outweighed by the more pressing need to prevent a vehicle overturning event. The potential for increased tire wear was also offset by unanticipated improvements in overall braking performance observed in testing, where even sliding inside wheels contributed to the deceleration of the vehicle during brake application (as compared to braking without application of inside wheel brakes in a two-channel system). Thus, whether the brakes are or are not being applied, a single-channel (single brake pressure modulator) braking system was found to provide effective roll detection and anti-roll braking performance comparable to previous two-channel systems, at lower cost and with a simpler system architecture.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.